Crystal Systems

For convenience of study and reference, crystals are divided into
six great systems, described by the comparative length and angular relation
of their crystallographic axes.

Cubic (or Isometric) System

A mineral is placed in the cubic system if it can be described by
three axes of equal length at right angles to one another. In a
cube, if axes were passed from the center of each face to the center
of the opposite face, the axes would be equal in length and at right
angles to one another. Diamond, spinel and garnet crystallize in
the cubic system.

Hexagonal System

(pronounced hex-AG-uh-nul) Hexagonal crystals have four axes, three of which
are equal in length and intersect at 600 angles. The fourth is perpendicular
to these and longer or shorter than the three previously described
axes, Ruby, sapphire, emerald and aquamarine crystallize in this
system.

Tetragonal System

(pronounced tech-TRAG-uh-nul) The tetragonal system is one in which
there are three axes that intersect at right angles, but only two
of which are equal in length, the third being either longer or shorter
than the first two. The basic form resembles a cube elongated in
one direction. Zircon crystallizes in this system.

Orthorhombic System

(pronounced or-tho-RO M-bik) This system is characterized by three
mutually perpendicular axes of unequal length. The basic form resembles
a box with length, width and depth unequal. Topaz crystallizes in
this system.

Monoclinic System

(pronounced mon-oh-KLIN-ik) The three axes in this system are unequal,
two intersect at an angle other than at right angles, and a third
is perpendicular to these two. The basic form can be visualized
as a box deformed. So that the top is still rectangular but one
side view is a parallelogram. Jadeite and nephrite crystallize in
this system.

Triclinic System

(pronounced try-KLIN-ik) The triclinic system is the one of the
least symmetry. It is described by three axes, all unequal in length,
and Inclined to one another at angles other than 900. A basic form
would resemble a box deformed so that all sides are parallelograms.
Labradorite and microcline feldspars crystallize in this system.

Crystal Axes

To describe a crystal form it is necessary to visualize the existence
of certain fixed lines of reference, similar to the imaginary line,
or axis, from the north to the south pole about which the earth rotates.
In an ideal crystal form these lines are of definite length in relation
to each other, extend in certain definite directions, and intersect
in the middle of the crystal at a point called the ORIGIN. Such imaginary
lines are called CRYSTAL AXES (the plural of axis). There must be at-least
three axes to describe a crystal, and in one case four are necessary.
These are indicated in the accompanying plate entitled "Models of the
Six Crystal Systems."

Forms Within Crystal Systems

Within each crystal system there are many different shapes that crystals
can take; each basic shape is called a CRYSTAL FORM. The form or combination
of forms most commonly taken by crystals of a given gem mineral is known
as the HABIT of that mineral. For example, the habit of garnet is the
DODECAHEDRON (pronounced doe-dek-uh-HEE-drun); of Zircon, a prism modified
by a pointed form called a BIPYRAMID, etc. (see accompanying plate entitled
"Examples of Crystal Forms"). For reasons related to the conditions
during growth, crystals of the same mineral may take different forms
or combinations of forms. For example, ruby and sapphire, varieties
of the same gem species, usually occur in crystals with different shapes.